122 H - JAMES-CLARK ON THE ANATOMY 



2d, a row of vibratile cilia (//), which lies immediately behind the veil; and 3d, a complex 

 '• adherent organ," in the form of a circle of centrifugal hooks (figs. 10, 17, 18, h) and 

 centripetal rays (/) which are firmly attached to the truncate, posterior face of the body. 



The velum (/,/') is merely an excessively thinned margin of the abruptly expanded, 

 truncate, circular base. It has a breadth which is at least one third as great as the length 

 of the vibratile cilia (g), which are attached in a single row immediately behind its basal 

 edge (fig. 17,/ 2 ). The free edge (f l ) of the velum is smooth and regularly curved. It is 

 not very difficult to distinguish from the closely set row of cilia (g) just posterior to it. 

 Although these cilia move so uniformly in concert, or in regular succession, as to appear 

 at times like a vibrating, frilled margin (fig. 10), yet, when they are nearly quiet, the veil 

 maybe distinctly seen — especially with a one-eighth of an inch objective — as an overlying, 

 separately undulating membrane. With oblique light, at about twenty degrees from direct 

 illumination, the velar edge is very conspicuous, and may be seen to be margined by a 

 thickening (fig. 17) which is easily traced across the whole width of the body, and at a 

 decidedly different focus from that in which the bases of the vibratile cilia underlie it. In 

 a profile view, it may be recognized as an abruptly terminating, marginal, tongue-like 

 projection, vibrating by fits and starts (fig. 11,/), at the periphery of the circular base. 



The basal vibratile cilia (g) form a complete, symmetrical circle about the truncate, pos- 

 terior end of the body. They are more delicate, and much longer than those of the 

 anterior, vibratory crown (b), and arise, in a single, closely set row, from a slightly projecting 

 annular ridge which immediately subtends the line of attachment of the velum. This 

 annular ridge, as will be seen presently, is the border (figs. 10, 17, l l ) of the adherent 

 organ. Owing to their excessive fineness, the close proximity in which they are set, and 

 the almost uniform succession with which one cilium follows the other in the series of 

 vibrations, this system gives to the unaccustomed eye the impression of an undulating, 

 fringe-like membrane, when it is viewed with only a moderate magnifying power; but with 

 an amplification of five hundred diameters, — if the objective be a good one, — one may 

 trace the cilia to their very bases, with the perfect confidence of not having seen amiss, and 

 at the same time satisfy himself conclusively that they are unequivocally distinct from the 

 veil which lies in front of them. There can be no hesitation, therefore, in pronouncing the 

 veil and the vibratile row of cilia as two distinct and separate systems, with no connection 

 whatever other than a close proximity of attachment to the basal margin of the body, and 

 their similar duties in the process of locomotion. 1 



The adherent organ (figs. 10, 17, h, i, I) is a complex apparatus, which altogether forms a 

 thin, circular disc, whose border (l l ) reaches to the margin of the base, or, in other words, 

 to the inner edge or line of attachment (/ 2 ) of the velum (/). 



About one third of the radius of the adherent organ, at the peripheral margin, is occu- 

 pied by a striated, annular membrane (I, l\ I 2 , 1% I*, / 5 ), which is separable from the rest of the 

 apparatus. It lies in front of the centrifugally projecting hooks (h), but closely pressed 

 against them, and extends centripetally (to I 3 ) as far as their bases. This membrane is 

 possessed of two sets of strice, which radiate from its inner to its outer margin. One set of 

 since occupy the anterior face (fig. 17, l l to l A ), and are comparatively quite coarse (/ 2 ), and 

 in number about ninety-six, i. e., four times the number of the hooks (//•) of this organ. 

 They lie wide apart, and are arranged so uniformly that two traverse the interval between 

 every two hooks, and two overlap every hook, where they run to the proximal margin (I s ) 



1 See the note ou the " adherent organ " at the end of this section, p. 124. 



